Re: [Phys-L] Suggestions for audio speakers

["John Clement" <clement@hal-pc.org>]

Of course the very low bass is severly limited with an open back, and the
small speakers have a fairly high resonance frequency.  This would need
severe equalization for good very low bass.  It is actually similar in some
ways to a commercial design, the Bose 901 (as I recall) which used 9 small
speakers, but then had an equalization box.  Unfortunately none of these are
considered portable, unless you use the army definition of portable has a
handle attached.  I think that most would consider portable to be something
that fits into a suitcase or duffel bag.  There is one portable monitor that
used to be made by Cambridge Sound Works.  It used a woofer in a closed
suitcase combined with two tweeters for stereo.   Unfortunately this does
not work for demonstrating how low frequency sounds can cancel when out of
phase, so it may not fill the bill for classroom demos.  Rensselaer used to
have students measure double slit interference using microwaves, light, and
sound in one lab.  They had 2 large loudspeakers separated about a meter or
so for the demo.  Waves should include all examples rather than separating
them by type of wave.

The problem with cone speakers is that the dispersion pattern changes with
frequency.  But wave guide speakers can achieve cylindrical waves, with some
variation in vertical dispersion.  Another Bose speaker which preceeded the
901 was designed to only sit in a corner of the room where 2 walls and
ceiling meet.  It was designed to simulate a pulsating sphere.  But I think
it never achieved much popularity because of the placement limitations.

John M. Clement
Houston, TX 

> On 2013, Mar 15, , at 13:40, "John Clement" 
> <clement@hal-pc.org> wrote:
>
> > Nobody seems to have addressed what you are asking for, so 
> here is an 
> > attempt to do this.
> > 1.  It is impossible to have speakers that have flat 
> response down to 
> > 20Hz that are also portable.  Basically you need large woofers to 
> > reproduce low frequency sounds.  Below the resonance 
> frequency of the 
> > cabinet/woofer system the sound cuts off sharply.  Ported speakers 
> > extend the low frequency, but then the cut off is even sharper.
>
>
> The engineer I previously mentioned designed semi-portable 
> systems w/ great value for the money.  A typical one 
> consisted of six oval (5 X 7") speakers (wired series 
> parallel) mounted on a baffle board and sides v. ~ a foot 
> (open back).  One placed it for "optimum sound".  A novel 
> innovation was to short the "nodes", so that one speaker's 
> resonance would be damped by its "brother".  If expensive 
> speakers were used, this wouldn't work!   He would demo A-B 
> with much more expensive ones -- Many would conclude not 
> worth the extra money. 
>
> Two advantages:  Good bass response from the large area, and 
> good mid range from the low mass --
>
> Another innovation was to rigidize the cone w/ a low density 
> building material whose composition a secret  I never 
> learned.  It looked like the white spray on sound absorber.  
> This, I suppose, improved the higher frequency response.  He 
> also experimented lowering woofer speaker resonance by using 
> glued on wire solder.  He also made3 acoustic suspension 
> speakers.  [ca 1956]
>
>
> bc former subscriber to speaker builder magazine.
>
> p.s.  All I saved from the Audio Amateur mag. was the drop 
> out detector article for my Sony PCM-F1.